Microphone with built-in preamp

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noizetoys

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I know that Chandler and Requisite have microphones that contain a pre/amplifier that outputs line level. I think it's an interesting idea but I haven't been able to find any info on any circuits for this sort of thing.

I'm really wanting to try out the idea on not only a condenser or tube mic, but also dynamic.

I'm wondering what a standard condenser would sound like if you didn't have to deal with an output transformer that is trying to push a mV signal down X number of feet to transformer balanced preamp. I would like to hear what an SM7B or 57 would sound like without all of the extra stuff in-between. Maybe better? Maybe worse? Maybe who cares?

I know that it would involve having a circuit to convert the impedance of the capsule and then a circuit to increase the gain. The gain stage would be followed by either a transformer or balanced driver chip/circuit. I don't think trying to run this off of 48V phantom is do-able or should even be a consideration. I would imagine treating it more like a tube mic with a separate supply would be the way to go.

As for tube mics, I would think using a dual triode or dual pentode might make it possible to have just a single tube handling impedance and gain, again using an external power supply.

Is this something anyone here has played with? I've looked here and a few other places and couldn't find any info discussing this approach other than the aforementioned companies.

Thoughts? Ideas? Suggestions? Notes?
 

Tubetec

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The Triton fethead runs off p48 , gives 20 db boost , theres a Klark teknik CM-1 which does the same but at a more agreeable price , not really a true line level output though as P48 is usually only available on mic inputs .
Ive thought about trying a proper tube based line level output mic , trouble is its hard to fit all the circuitry and transformer required into a mic body , then theres also the question of heat disipation in a very confined space . Ive been thinking about making a tube condenser mic with a cathode follower for impedence conversion in the mic body itself , then a line output stage built into the PSU .

If its dynamic or ribbon mics your using I'd consider trying the KT CT-1 , its around 22 euros .
 

Gus

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DIY CF with output transformer in in power supply

IIRC amps in power supplies have been done I think there was a Gefell with a amp in the PS

As Tubetec posted there are phantom powered P48 add on amps. There are DIY threads at this forum

WHY would you want a preamp in the microphone? There is an amp in condenser microphones, followers and voltage gain circuits

A common condenser microphone tube circuit has a gain stage to a CF you could up the CF drive and use a lower ratio transformer for more output.

You can add more gain to microphone circuits, also some microphones have pads.

With a preamp in a microphone the controls are now at the microphone and you would need to go to the microphone to adjust setting(digital remote control will cause extra work with possible interference as well as FCC issues if you are selling the microphone)

The singer now has access to the controls do you want that?

How much signal level do you need anyway
 

Tubetec

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The Royer tube mics are a great starting point for anyone who wants to try a tube based design .
I noticed also in the small capsule condenser he moved both the coupling cap and cathode resistor to the PSU to save space but thats also how its done in the B&K cathode follower mics , because the signal cable to the psu is within the feedback loop of the cathode follower loses in the cable due to capicitance are minimised , in fact B&K quote a bandwidth of 200 khz for their design head amplifier .

The Royer uses an input transformer back to front as output . Adding an extra output stage and associated transformer in the psu might not add much to the overall cost and give higher drive capability , downside is you will have to wander off into the studio to adjust gain .

Ive been thinking about trying a tube based long tail pair as output driver , push pull ac coupling to the transformer ,then feedback from transformer secondary back to one of the grids , similar to the BBC designed head amps/ output stage used with their modified AKG mics .
 

RuudNL

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IMHO you would need 'some' form of gain control on the microphone, be it a knob on the microphone itself, or a form of remote control.
How else would you compensate for the output levels of a (careless) whisper and a loud rock singer? The dynamic range could be a problem in these cases.
 
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Tubetec

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Gain control control of the output stage by varying NFB could work at the PSU end , not a whole lot better than varying it at the mic end , but alleviates the need to cram the CF ,output stage and transformer into the mic body .
I guess a relay attenuator board could be used if remote capability was a requirement , that has downsides too , clicking relays and extra fuss sending a control signal from control room to studio etc .
 

noizetoys

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Thanks for your replies. Interesting takes.

In-line boosters are a fix for an issue I'm not trying to overcome. I'm not trying to overcome the lack of gain in some mics, I'm trying to overcome having all of the misc. circuitry between the output of the capsule and the input of a recorder. Really more of an experiment to see if shortening the distance between capsule and amplifier would make a significant if any difference in the sound.

I have the several of the Royer Mod mics and a boat load of 5840's so I'm looking to see what could be adjusted on that circuit. It's a good circuit, sounds 'tubey' without sounding 'bad tubey'. I also agree that it's a great starting point for learning. David Royer is even reachable on the phone and doesn't seem to have any issue answering technical questions. But not too many, he does have things to do.

Gain control would be of concern, but I'll burn that bridge when I get to it.

I no longer do commercial recording so I'm not worried about the idea of having to get up and make an adjustment to the mic (or PSU) if needed. And NO, I would never want to give a tiger any direct control over their mic. I don't see that turning out well at all.

I have no interest in making a commercial available product. I have a great deal of respect for anyone making and selling microphones in this rush-to-the-bottom clone market. I'm trying to learn a bit more and see if there is any benefit to having the shortest signal path possible.

It may very well turn out that it is a very bad idea and it makes not audible difference or that the difference is not at all pleasing.

Inquiring minds want to know. And I'm a little curious too!
 

emrr

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There was a max output level test in a recording mag maybe 20 years ago, and a number of modern condensers already do output line level signals if faced with a loud source. In which case things are already as they should be. That suggests any variable gain would have to be capable of unity gain. For myself, I don't want to be in the control room with the gain control on a mic I can't get to for tweaks. Soundcheck sounds like a nightmare.
 

Gus

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might be a picture of the insides page 57
 

Tubetec

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Rupert Neve designed remotely controlled preamps for the Air Monserrat console , cost was probably no barrier there though .
Bo Deadly here has been toying with the idea recently and I think JR may have way back but abandoned it in the end .
 

Khron

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For what it's worth...

Thus summer, since i moved to a bigger city here, i got together a new band. We're still in the jamming stage, so i decided to record our stuff with a few mics and an older interface I had lying around at home. Now, since my old MOTU 828mk2 only has two mic preamps (one of which went to the guitar cab mic), i was "forced" to experiment / follow a hunch, in order to be able to use more channels.

I can confirm that even dynamic mics can put out levels at least in the ballpark of line-level. Granted, snare mics are not exactly facing a "typical" SPL, and the 828mk2 has selectable -10dBv/+4dBu line inputs, as well as optional 6dB boost post-ADC, but still...
 
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